Method for generating a lens position signal and corresponding apparatus for reading from and/or writing to an optical recording medium

ABSTRACT

In order to generate a lens position signal (LCE), which describes the position of the optical axis of an objective lens ( 6 ) of an apparatus for reading from and/or writing to an optical recording medium ( 7 ) with regard to the optical axis of the remaining components ( 2, 3, 4, 5, 8, 9 ) contained in an optical scanner ( 21 ), the application of the DPP method is proposed in accordance with a first exemplary embodiment, wherein a primary-beam error signal (CPP) and a secondary-beam error signal (OPP) are obtained with the aid of the DPP method, wherein the desired lens position signal (LCE) is generated by addition of the primary-beam error signal (CPP) and the secondary-beam error signal (OPP). In accordance with a second exemplary embodiment, it is not necessary to generate a primary beam, rather it suffices to detect the secondary beams reflected from the optical recording medium ( 7 ) in order to obtain the lens position signal (LCE) by addition of the secondary-beam error signals (OPP 1,  OPP 2 ) generated in a manner dependent thereon.

[0001] The present invention relates to a method for generating a lensposition signal, which describes the position of the optical axis of anobjective lens of an apparatus for reading from and/or writing to anoptical recording medium with regard to the optical axis of an opticalscanner used in this apparatus, and also to a correspondingly configuredapparatus for reading from and/or writing to an optical recordingmedium.

[0002] A track error signal is conventionally generated in apparatusesfor reading from and/or writing to optical recording media, such as, forexample, optical recording media (e.g. DVD-RAM) in which informationtracks are contained both in depressions (G), designated as “groove”,and in elevations (L), designated as “land”, which track error signalcan be used for tracking regulation in the respective apparatus. One ofthe widespread methods for forming the track error signal is theso-called “differential push-pull” (DPP) method, as is described forexample in EP 0 745 982 A2. In this case, the laser beam output by alaser diode is split into three beams, namely a primary beam and twosecondary beams which scan mutually adjacent tracks of the opticalrecording medium respectively used. The primary and secondary beamsreflected from the optical recording medium are evaluated in order toobtain, in a manner dependent thereon, primary-beam and secondary-beamtrack error signals from which the desired track error signal isgenerated by means of weighted combination.

[0003] A corresponding arrangement is illustrated by way of example inFIG. 8. The light emitted by a light source or a laser 1 passes througha collimator lens 2 and is then split into the primary beam (i.e. a0th-order beam) and the two secondary beams (i.e. ±1st-order beams) by adiffraction grating 3. The primary beam, which reads the information tobe scanned in a track of a corresponding recording medium 7, usuallycontains the majority (approximately 80-90%) of the light information.The two secondary beams each contain the remaining 5-10% of the totallight intensity, it being assumed for the sake of simplicity that thelight energy of the higher orders of diffraction of the diffractiongrating 3 is zero. These three beams are focused onto the opticalrecording medium 7 via a polarizing beam splitter 4 and a quarter-waveplate 5 and also an objective lens 6, in order to read from and/or writeto the said optical recording medium. The three beams reflected from theoptical recording medium 7 are fed via the beam splitter 4 and acylindrical lens 8 to a photodetector unit 9, which detects the threebeams reflected from the optical recording medium 7. The three beams areindicated symbolically in the figure between cylindrical lens 8 andphotodetector unit 9. Connected to the photodetector unit 9 is anevaluation unit 10, which evaluates the detected signals of thereflected primary and secondary beams for the purpose of generating atrack error signal.

[0004] The diffraction grating 3 is incorporated in such a way that theimaging of the two secondary beams scans precisely the centre of thesecondary tracks or (in the case of media which can be written to onlyin “groove” tracks) the centre beside the track scanned by the primarybeam. Since the secondary beams and the primary beam are intended to beoptically separable from one another, the positions of their imaging onthe optical recording medium 7 and on the photodetector unit 9 areseparate from one another. If the optical recording medium 7 rotates,then one of the secondary beams is situated in front of, and the othersecondary beam behind, the primary beam in the reading or writingdirection. The evaluation unit 10 of the arrangement shown in FIG. 8evaluates the light intensities reflected onto the photodetector 9separately for each of the three beams.

[0005] In the evaluation unit 10, the detected signals both of theprimary beam and of the secondary beams are used to generate, consideredby themselves in each case, a push-pull signal which represents thetrack error of the respective beam with respect to the track. However,since the two secondary beams scan the secondary tracks with respect tothe read/write track, their push-pull track error is inverted withrespect to that of the primary beam. Considered by themselves, therespective push-pull components thus contain the actual track error withrespect to the respectively scanned track. Since the track position ofthe three beams can only change together, the three push-pull signalschange equally.

[0006] The objective lens 6 of an optical scanner 21 as sketched in FIG.8 must be mounted in a movable manner in order, even in the case of anoptical recording medium 7 which has vertical wobble and/oreccentricity, to make it possible to focus the scanning beam and keep iton a predetermined track. That part of the scanner 21 which comprisesthe elements 2, 3, 4, 5, 8, 9 defines an optical axis 22. The objectivelens 6 is arranged in its rest position ideally in such a way that itsoptical axis 23 corresponds to the optical axis 22 of the other opticalcomponents of the optical scanner 21.

[0007] The movement of the objective lens 6 is usually achieved by meansof an electromagnetic drive. In this case, the objective lens is kept ina predetermined rest position by an arrangement of articulated joints orsprings, from which position it can be deflected from its rest positionby application of a current to the electromagnetic drive. To that end,the output signals of the evaluation unit 10 provide track error andfocus error signals which encompass the position of the objective lens 6and correct it with the aid of regulating circuits.

[0008] If the intention is to scan an optical recording medium 7 whosetracks are applied in spiral form, then the objective lens 6 isdeflected to an increasing extent during a continuous scanningoperation. Its optical axis 23 is therefore displaced increasingly farfrom the optical axis 22 of the other optical components. In order tocounteract this displacement of the optical axes with respect to oneanother, provision is usually made of a servo or linear motor whichsubsequently shifts the scanner 21 with the optical components 2, 3, 4,5, 8, 9 incorporated therein in such a way that the optical axes deviatefrom one another as little as possible. This motor is usually referredto as coarse track motor. According to the prior art, the drivingvoltage of the electromagnetic drive of the objective lens is used as acriterion for the deviations of the optical axes and the coarse trackmotor is driven in such a way that the driving voltage tends to zero.

[0009] For this purpose, provision is made of a further regulatingcircuit, which ensures that the optical axes 22, 23 of scanner 21 andobjective lens 6 correspond. According to the prior art, the drivingvoltage of the electromagnetic drive of the objective lens 6 isevaluated for this purpose. In this case, it is assumed that the opticalaxis 23 of the objective lens 6 does not deviate from the axis of theother components 22 when the drive coils are de-energized. Since theobjective lens is suspended in a resilient fashion, this assumption isnot correct in all operating situations. By way of example, theobjective lens changes its position even without driving of the drivecoils if external forces act on it, as may occur in the event of animpact against the player. Furthermore, ageing of the articulated jointsor springs may cause the rest position of the objective lens to bechanged such that the optical axes deviate from one another. Theseeffects cannot be detected from the driving voltage of the drive coils.

[0010] If the objective lens 6 is then moved for example during a trackjump, the imaging of the primary and secondary beams on thephotodetector unit 9 also moves. This displacement of the imagingresults in an offset voltage at the output of the evaluation unit 10,the direction of this offset voltage being identical for all of thebeams.

[0011] The displacement of the objective lens 6 thus gives rise to anoffset voltage which does not originate from an actual track error andis therefore an interference. The genuine track error component and theundesirable lens-movement-dependent component are added in the push-pullsignal which is detected by the respective detectors of thephotodetector unit 9 and yielded by the evaluation unit 10.

[0012] If the push-pull signals of the secondary beams are then addedand this sum is subtracted from the push-pull signal of the primarybeam, this undesirable lens-movement-dependence component is cancelledout given appropriate weighting between the primary and secondary beamcomponents. By contrast, since the push-pull components of primary andsecondary beams are inverted with respect to one another, they are addedin the correct phase after application of the subtraction, with theresult that, given correct setting of the weighting factor, the actualtrack error is obtained. A method for determining a suitable weightingfactor is described by way of example in EP 0 708 961 B1.

[0013] From the previously described properties of the conventional DPPmethod, it is apparent that, owing to the position of the secondarybeams, the phase shift between the primary beam and the secondary beamsis nominally 180 degrees. This is advantageous since, as a result of thedifference formation, the track error components of the primary beam andof the secondary beams are added with the largest possible amplitude. Ifthe position of the beams on the tracks is considered, then the angle ofthe diffraction grating 3 is set, for achieving the maximum amplitude ofthe track error signal, precisely in such a way that (for example in thecase of a DVD-RAM) the secondary beams impinge on the track centres ofthe secondary tracks or (in the case of media which can be written toonly in “groove” tracks) precisely on the region between two tracksbeside the track scanned by the primary beam.

[0014] The aim of the DPP method described previously is to form a trackerror signal which has no offset dependence on the position of theobjective lens 6 relative to the optical axis of the scannerrespectively used. In the case of the previously described combinationof the push-pull components of the primary beam and of the secondarybeams, although the actual track error can be obtained, owing to thecancellation of the lens-movement-dependence component it is nonethelessnot possible to detect the position of the objective lens 6 with regardto the optical axis of the scanner.

[0015] During a track following operation, the objective lens 6 isdisplaced perpendicularly to the track direction of the opticalrecording medium 7, i.e. the optical axis of the objective lens 6 ismoved away from the optical axis of the scanner 21. This results in acorresponding displacement of the imaging of the reflected scanning beamon the detector elements of the photodetector unit 9. While therespectively scanned track is followed correctly, the evaluation unit 10cannot recognize in this case that the optical axes of objective lens 6and scanner 21 do not correspond. For this reason, it is necessary, inprinciple, to provide a signal which describes the position of theobjective lens 6 with regard to the optical axis 22 of the scanner 21.

[0016] It is furthermore advantageous, during a positioning operation,as is necessary for example for an access to another piece of music on aCD, to provide for the control unit of the apparatus auxiliary signalswhich enable a fast access to the piece of music desired by the user ofthe apparatus.

[0017] An object of the present invention is to propose a method forgenerating a lens position signal, which describes the position of theobjective lens with regard to the optical axis of an optical scanner,and also a corresponding apparatus for reading from and/or writing to anoptical recording medium.

[0018] Furthermore, possibilities for generating auxiliary signals foran improved track jump are shown.

[0019] This object is achieved according to the invention by means of amethod having the features of claim 1 or 11 and an apparatus having thefeatures of claim 20 or 27. The subclaims each define preferred andadvantageous embodiments of the present invention.

[0020] A first exemplary embodiment of the present invention proposesthat the lens position signal be generated by employing the DPP methoddescribed in the introduction. In contrast to the prior art described inthe introduction, however, the push-pull signal of the secondary beamsis added to the push-pull signal of the primary beam in order to obtainthe lens-movement-dependent component. In this case, in particular, aweighted addition is carried out, in which case the weighting factor canbe set to an ideal value in a manner dependent on the distance betweenthe two secondary beams and the primary beam and on the track spacing.In a variant of the first exemplary embodiment, a normalization of thesignals derived from the beams used is provided in order to simplify thesetting of the weighting factor.

[0021] In accordance with a second exemplary embodiment of the presentinvention, the lens position signal is derived directly from thepush-pull signals of the secondary beams, i.e. the push-pull signal ofthe primary beam is in this case not included in the generation of thelens position signal. In this case, it is particularly advantageous ifthe secondary beams are imaged on the optical recording medium inaccordance with the following formula: $\begin{matrix}{{{\Delta \quad x} = {{\left( {{2n} - 1} \right)*\frac{p}{2}\quad {where}\quad n} = 0}},1,2,\ldots} & (1)\end{matrix}$

[0022] In this case, Δx designates the distance between the secondarybeams and the (imaginary or existing) primary beam and p designates thetrack spacing. In the latter, the track error components of thepush-pull signals of the two secondary beams cancel one another out, sothat the resultant summation signal only comprises thelens-movement-dependent contribution and, consequently, corresponds tothe desired lens position signal, which can be used for example in theevent of a track jump for stabilization of the actuator.

[0023] In the case of the previously described orientation of thesecondary beams, it is additionally possible to generate a directionsignal from the phase between the difference signal of the two push-pullsignals of the secondary beams and the push-pull signal of the primarybeam. Equally, it is possible to generate a track error signal.

[0024] The present invention is explained in more detail below usingpreferred exemplary embodiments with reference to the accompanyingdrawings.

[0025]FIG. 1 shows a first exemplary embodiment of the present inventionfor generating a lens position signal,

[0026]FIG. 2 shows a variant of the first exemplary embodiment shown inFIG. 1,

[0027]FIG. 3 shows a further variant of the first exemplary embodimentshown in FIG. 1,

[0028]FIG. 4 shows a track image with the beam arrangement of theprimary beam and secondary beams and the push-pull signals obtained withthis beam arrangement in accordance with a second exemplary embodimentof the present invention,

[0029]FIG. 5 shows the second exemplary embodiment of the presentinvention for generating a lens position signal,

[0030]FIG. 6 shows a beam arrangement of a primary beam and foursecondary beams and also the push-pull signals obtained with this beamarrangement with a variant of the second exemplary embodiment,

[0031]FIG. 7 shows, by way of example, a photodetector unit fordetecting the reflected primary and secondary beams shown in FIG. 6,

[0032]FIG. 8 shows a simplified construction of an optical scanner forcarrying out the DPP method according to the prior art, and thisconstruction can also be applied to the present invention, and

[0033]FIG. 9 to

[0034]FIG. 12 show further variants of the first exemplary embodimentshown in FIG. 3, a normalization being provided.

[0035] As was described in the introduction, the track error signalgenerated in accordance with the DPP method is composed of thecorresponding component of the primary beam and the added components ofthe secondary beams, in accordance with the prior art the components ofthe secondary beams being added and the resultant sum being subtractedfrom the component of the primary beam with appropriate weighting.

[0036] For all of the following considerations, it is assumed in asimplification that the intensities of the three scanning beamsconsidered are identical when impinging on the photodetector unit 9. Inpractice, however, the intensity of the secondary beams is dependent ontheir track position, on the reflection of the scanned track and also onthe properties of the diffraction grating 3 and is weaker than theintensity of the primary beam, so that the intensity of the secondarybeams has to be scaled correspondingly with respect to the primary beamintensity. This can ideally be done by means of a normalization.

[0037] Under the above-described assumption, the following relationshipshold true; in this respect, also see, for example, FIG. 4 described inmore detail further below:

DPP=CPP−K*OPP  (2) $\begin{matrix}{{CPP} = {{a*{\sin \left( {2\pi*\frac{x}{2p}} \right)}} + {kl}}} & (3) \\{{OPP} = {{{a*\left( {{\sin \left( {2\pi*\frac{x + {\Delta \quad x}}{2p}} \right)} + {\sin \left( {2\pi*\frac{x - {\Delta \quad x}}{2p}} \right)}} \right)} + {k\left( {l + l} \right)}}\quad = {{a*\left( {{\sin \left( {2\pi*\frac{x + {\Delta \quad x}}{2p}} \right)} + {\sin \left( {2\pi*\frac{x - {\Delta \quad x}}{2p}} \right)}} \right)} + {2{kl}}}}} & (4)\end{matrix}$

[0038] In this case, DPP designates the signal obtained in accordancewith the DPP method, CPP designates the corresponding component of theprimary beam, OPP designates the component of the secondary beams, Kdesignates a weighting factor, x designates the scanning position of abeam relative to the track centre, Δx designates the distance betweenthe two secondary beams and the primary beam and p designates the trackspacing, which in this case, corresponding to the definition inaccordance with the DVD-RAM standard, is measured between the centres oftwo adjacent tracks. l designates the movement of the objective lens 6from the rest position. The amplitudes a and k are factors which dependon the geometry of the scanned tracks, the sensitivity of thephotodetector unit 9, etc. Since the three beams are mechanicallycoupled to one another, the variables x and l in the formulae for theCPP signal and the OPP signal are identical in each case.

[0039] In order to achieve compensation of the lens-movement-dependentcomponent l, the following equation must be satisfied:

DPP _(l) =CPP _(l) −K*OPP _(l)≡0  (5)

[0040] In this case, the index “l” respectively designates thelens-movement-dependent component of the corresponding signal. Takingaccount of formulae (3) and (4) above, it follows for the weightingfactor for compensation of the lens-movement-dependent component that:

K=0.5  (6)

[0041] This weighting factor K is independent of the orientation of thesecondary beams with regard to the primary beam. It is usually attemptedto make the track error amplitude maximum by setting the distance Δxaccordingly. This is achieved, in the case of evaluation of formulae (2)to (4) above with K=0.5, when the following relationship holds true:$\begin{matrix}{{\cos \left( {\pi*\frac{\Delta \quad x}{p}} \right)} = {- 1}} & (7)\end{matrix}$

[0042] Since the cosine function is periodic, this holds true for:

Δx=(2n+1)*p where n=0, 1, 2  (8)

[0043] It follows from formulae (2) to (4) that when a new weightingfactor G with a negative sign is used, i.e. when the subtraction of theOPP signal from the CPP signal is replaced by an addition of these twosignals, only the lens-movement-dependent component is retained, whilethe individual track error components cancel one another out. Inparticular, the following relationship must hold true for compensationof the track error components:

DPP _(x) =CPP _(x) −−G*OPP _(x≡)0  (9)

[0044] In this case, the index “x” designates the track-error-dependentcomponent of the respective signal. The relationship of (9) is satisfiedtaking account of relationships (3) and (4) above if the following holdstrue: $\begin{matrix}{{DPP}_{x} = {{a*{\sin \left( {\pi*\frac{x}{p}} \right)}*\left( {1 - {2G\quad {\cos \left( {\pi*\frac{\Delta \quad x}{p}} \right)}}} \right)} \equiv 0}} & (10)\end{matrix}$

[0045] The track-error-dependent component of the DPP signal can thus beeliminated in a manner dependent on Δx and p if the following holdstrue: $\begin{matrix}{{1 - {2G\quad {\cos \left( {\pi*\frac{\Delta \quad x}{p}} \right)}}} = 0} & (11)\end{matrix}$

[0046] Given an assumed distance between the secondary beams and theprimary beam of Δx=p, the following results in this respect:

G=−0.5  (12)

[0047] From the negative sign of the weighting factor G in accordancewith formula (12), it emerges that the subtraction must be replaced byan addition. If the secondary beams are arranged at Δx=p, theapplication of the addition of the CPP and OPP signals thus suffices tomake the track error components tend to zero and to obtain thelens-movement-dependent component. With G=−0.5, by inserting Δx=p intoformulae (2) to (4), the lens-movement-dependence component is obtainedas follows:

DPP _(l)=2kl  (13)

[0048] The signal thus obtained contains only thelens-movement-dependent component; it is designated by LCE (Lens CentreError).

[0049]FIG. 1 illustrates a corresponding exemplary embodiment forgenerating the lens-movement-dependent component for the correspondinglens position signal LCE by employing the DPP method. In this case, itis assumed that the photodetector unit 9 for detecting the reflectedprimary beam has a photodetector unit 12 having four photosensitiveareas A-D, while respective photodetector elements 11, 13 having onlytwo photosensitive areas E1, E2 and F1, F2, respectively, are providedfor detecting the reflected secondary beams. As can be seen from FIG. 1,provision is made of an amplifier having a gain factor −1 and, in orderto change over from track regulation operation to lens position control,a switch, the signals CPP and 0.5·OPP being either subtracted or addeddepending on the switch position.

[0050] In order to be able to measure the lens position during theplayback operation of the optical recording medium 7, it is necessarysimultaneously to form the track error signal DPP from the differenceand also the lens position signal LCE from the sum of the partialsignals CPP and OPP. FIG. 2 shows a corresponding variant of theexemplary embodiment shown in FIG. 1. Since both signals are availablesimultaneously in this case, the track regulating circuit can be closedand, at the same time, the information about the lens position can beused for readjusting the coarse track motor of the optical scanner 21.

[0051] If the distance Δx between the secondary beams and the primarybeam is not Δx=p but rather Δx=3/4p, for example, the following resultsfor the weighting factor G, which leads to the compensation of the trackerror component, in accordance with formula (11): $\begin{matrix}{G = {- \frac{1}{\sqrt{2}}}} & (14)\end{matrix}$

[0052] In this case, the optimum weighting factor K for generating thetrack error signal differs from the weighting factor G required forgenerating the lens position signal not only in terms of sign but alsoin terms of magnitude. The weighting factor K for suppressing thelens-movement-dependent component is ideally always 0.5, while theweighting factor for compensation of the track error component is alwaysnegative, but is to be adapted to the position of the secondary beams.Accordingly, the arrangement shown in FIG. 2 can be modified as shown inFIG. 3, in which case the weighting factor used for generating the lensposition signal LCE can be set in a variable manner.

[0053] If a variably adjustable weighting factor is provided, secondarytrack distances Δx different from those mentioned above can also be usedin conjunction with the DPP track error method. Track distances in therange of p/2<Δx<3p/2 can theoretically be utilized. The limits p/2 and(3/2)*p cannot be utilized in practice since the track errorcontribution in the signal component OPP becomes zero here and, evengiven an infinitely large factor G set, compensation of the track errorcontribution of the CPP signal could not be achieved.

[0054] Instead, it is possible here to use the sum of the signals OPP1and OPP2 alone in order to obtain a lens position signal. This isillustrated in the following section and also in FIGS. 4 and 5. If theformation of a track error signal according to the DPP method isdisposed with, then the lens position signal can also be formed witharbitrary secondary track distances Ax.

[0055] Limiting cases emerge here as Δx=0 or Δx=2·n·p, since thetrack-error-dependent components of the partial signals CPP, OPP1 andOPP2 are in phase here and compensation of these components cannot beachieved.

[0056] Furthermore, it should be noted that the sign of the weightingfactor G is reversed for 0<Δx<p/2 and also for 3p/2<Δx<2p.

[0057] The previously described method for generating the lens positionsignal LCE using the DPP method is suitable, in particular, for alloptical recording media which, on account of their physicalconstruction, are suitable for application of the DPP method if, at thesame time, a track error signal is intended to be generated. However,the position of the optical axis of an objective lens with regard to theoptical axis of an optical scanner can also be ascertained if theoptical recording medium 7 is not suitable for application of the DPPmethod. Corresponding examples of this will be explained below.

[0058] The lens position signal LCE can be formed, for example, inaccordance with a further exemplary embodiment, if the secondary beamsare imaged with the following distance Δx: $\begin{matrix}{{{\Delta \quad x} = {{\left( {{2n} - 1} \right)*\frac{p}{2}\quad {where}\quad n} = 0}},1,2,\ldots} & (15)\end{matrix}$

[0059] The consequence of this is that the track error components of thetwo secondary beams OPP1=(E2−E1) and OPP2=(F2−F1) cancel one another outand their magnitude with respect to the track error signal becomes zero.This means that the summation signal OPP yields only the contributiondependent on the lens movement l and, consequently, corresponds directlyto the lens position signal LCE.

[0060] As is shown in FIG. 4, a scanner which uses a three-beam trackerror signal for track regulation typically has two secondary beams 15,16, which are imaged on the optical recording medium with Δx=p/2 besidethe primary beam 14. In this case, Δx is measured in the direction ofthe imaginary x-axis depicted, whose origin lies in the centre of thescanning spot of the primary beam 14, in this case in the centre of atrack. As is also shown in FIGS. 1 to 3, the photodetectors used fordetecting the reflected secondary beams 15, 16 are in each case dividedinto two parts in order simultaneously to obtain a track error signaland a lens position signal. The separation of the photodetector areas issuch that the beams reflected from the optical recording medium impingesymmetrically with respect to the separating line.

[0061]FIG. 4 illustrates a track image for such a beam arrangement ofthe secondary beams with Δx=p/2 and also the resultant track errorsignals. A corresponding arrangement for obtaining the signalsillustrated in FIG. 4 is shown in FIG. 5.

[0062] As can be seen from FIG. 4, the track error components of the twosecondary beam signals OPP1 and OPP2 cancel one another out, so that thesummation signal OPP=OPP1+OPP2 yields only the contribution dependent onthe lens movement l and, consequently, corresponds to the desired lensposition signal LCE. With this orientation of the secondary beams, it isadditionally possible to generate a direction signal DIR from the phasebetween the difference signal OPP1−OPP2 and the CPP signal with the aidof a phase comparator shown in FIG. 5, since the phase of these twosignals with respect to one another is +90° or −90° depending on thedirection of movement. A track error signal TE is likewise availablewhich, however, has only half the amplitude of the ideal DPP signal.Moreover, as is shown in FIG. 5, it is possible to obtain a so-called“Track Zero Cross” signal TZC and also the information of what type oftrack (groove or land) the scanning beam is presently scanning.

[0063] The track zero cross signal TZC is obtained from the signal CPPby means of a comparator in FIG. 5. Instead of this, as an alternativeit may also be obtained from the corrected signal DPP. Anotheralternative, not illustrated here, provides for only one of the signalsOPP1 or OPP2 to be used, instead of the difference OPP1−OPP2. Thisobviates the difference formation; the signal fed to the comparator thenhas only half the amplitude, as can be seen from FIG. 4.

[0064] By means of a corresponding configuration of the diffractiongrating 3 shown in FIG. 8, it is also possible to direct onto theoptical recording medium 7 only two or, alternatively, more than threebeams in such a way that at least one of the beams impinges on a“groove” track and generates a corresponding track error signal, whileanother beam impinges on a “land” track and likewise generates acorresponding track error signal which is phase-shifted through 180°with respect to the track error signal of the first-mentioned beam. Ifthese two signals are added to one another, then the track errorcomponent contained therein is likewise cancelled out, and all thatremains is the component dependent on the lens movement l of theobjective lens 6.

[0065] Furthermore, the present invention can also be applied toscanners with holographic optical components, provided that two(secondary) beams are generated which impinge on the optical recordingmedium 7 at a distance of Δx=(2n-1)·p/2 from an (imaginary or existing)primary beam and whose imaging on a correspondingly configuredphotodetector unit 9 or an evaluation unit 10 generates two push-pullsignals which have a phase shift of 180° with regard to their trackerror component, provided that the components proportional to the lensmovement are added in the correct phase.

[0066] This is realized for example in a five-beam scanner, the±1st-order secondary beams in each case impinging on the edges between a“groove” and a “land” track, while the ±2nd-order secondary beamsimpinge on the track centres of the adjacent tracks of the primary beam.A corresponding track image is illustrated in FIG. 6, the primary beamagain being provided with the reference symbol 14, the 1st-ordersecondary beams being provided with the reference symbols 15, 16 and the2nd-order secondary beams being provided with the reference symbols 17,18. The distance between the 1st-order secondary beams 15, 16 and theprimary beam 14 is thus Δx1=p/2, while the distance between the2nd-order secondary beams 17, 18 and the primary beam 14 is Δx2=p.

[0067]FIG. 7 illustrates an exemplary embodiment of the individualphotodetectors 11-13 and 19, 20 of the photodetector unit, which eachserve for detecting a beam 14-18 reflected from the correspondingoptical recording medium. In this case, the primary beam 14 is detectedby a photodetector element 12 having four light-sensitive areas A-D,while the secondary beams are in each case detected by photodetectorelements 11, 13, 19 and 20, respectively, having two light-sensitiveareas E1, E2, F1, F2, G1, G2 and H1, H2, respectively. From the outputsignals of the light-sensitive areas of the individual photodetectorelements, the following push-pull signals are again determined by anevaluation unit, which signals are illustrated by way of example in thelower part of FIG. 6:

CPP=(A+D)−(B+C)  (16)

OPP 1=E 2−E 1  (17)

OPP 2=F 2−F 1  (18)

OPP 3=G 2−G 1  (19)

OPP 4=H 2−H 1  (20)

[0068] A DPP signal is formed for example from the difference betweenthe signal of the primary beam, i.e. the CPP signal, and the summationsignal of the 2nd-order secondary beams, as follows:

DPP=CPP−K*(OPP 3+OPP 4)  (21)

[0069] The sum of the push-pull signals of the two first-order secondarybeams again yields a voltage which is proportional to the lens movementl of the objective lens, without a track error contribution, since thetrack error components, as described above, cancel one another out, sothat the desired lens position signal can be derived directly from thissummation signal:

LCE=OPP 1+OPP 2  (22)

[0070] In addition, a direction signal indicating the direction withwhich the scanning beam crosses the tracks with the track regulatingcircuit open can be derived from the phase relationship of the push-pullsignal of one of the secondary beams 15-18 with respect to the push-pullsignal of the primary beam 14.

[0071] As has already been mentioned in the introduction, for all theabove considerations, it was assumed in a simplification that theintensities of the three scanning beams considered are identical whenimpinging on the photodetector unit 9. The compensation factors G and Krespectively specified therefore apply only if this simplification isemployed.

[0072] In practice, however, the intensity of the secondary beams isdependent on their track position, on the reflection of the scannedtrack and also on the properties of the optical diffraction grating 3and is weaker than the intensity of the primary beam, so that theintensity of the secondary beams has to be scaled correspondingly withrespect to the primary beam intensity. Ideally, this is done by means ofa normalization. To that end, the signals derived from the reflectedbeams are normalized. The signals CPP and OPP or, as an alternative, theindividual signals OPP1 and OPP2 are normalized by dividing thesesignals by the summation signals which are proportional to the quantityof light respectively taken up by the detector areas. Such anormalization is realized for example in the evaluation unit 10.

[0073] Proceeding from the exemplary embodiment shown in FIG. 3, FIGS. 9and 10 show two variants of a normalization. FIG. 9 shows an exemplaryembodiment of a normalization in each case for the primary beam (CPP)and jointly for the secondary beams (OPP). In this case, the normalizedsignals are designated as CPPN, OPPN, LCEN and DPPN using an appended“N”. FIG. 10 shows an exemplary embodiment in which the push-pullcomponents of the three beams are normalized separately before thesignals LCE and DPP are formed therefrom by weighted addition andsubtraction, respectively.

[0074] As described above, it is necessary to adapt the weighting factorG to the secondary track spacings. By way of example, if the variantshown in FIG. 9 is taken as a basis, then the signal amplitude of thesignal LCE is dependent on the setting of the compensation factor G.This is avoided by a further variant of the variants shown in FIGS. 9and 10, which is described below.

[0075] The variant shown in FIGS. 11 and 12, respectively, relates tothe weighting between primary beam and secondary beams. It isadvantageous, for example, to replace the weighting factor G for thesecondary beam signal by two weighting factors G′ and 1-G′, which act onthe primary and secondary beam signals, where G′ can be calculated fromG according to the following relationships: $\begin{matrix}{G^{\prime} = \frac{G}{\left( {1 + G} \right)}} & (23)\end{matrix}$

[0076] What is achieved by splitting the weighting factor G into twoweighting factors dependent on G′ is that the amplitude of thelens-movement-dependent signal LCE is independent of the respectiveweighting factor to be set. In an analogous manner, formula (23) canalso be applied to the weighting factor K for forming the DPP signal.The factors G and K are chosen for example analogously to FIG. 9 andFIG. 10, respectively. The signals weighted in this way are designatedby LCEN′ and DPPN′.

1) method for generating a lens position signal, wherein the lensposition signal (LCE) describes the position of the optical axis (23) ofan objective lens (6) of an apparatus for reading from and/or writing toan optical recording medium (7) with regard to the optical axis (22) ofan optical scanner (21) assigned to the objective lens (6), whereinprimary and secondary scanning beams (14-18) incident on adjacent tracksof the recording medium (7) are generated and the primary and secondaryscanning beams reflected from the recording medium (7) are detected, andwherein a primary-beam error signal (CPP) and a secondary-beam errorsignal (OPP) are derived from the detected reflected primary andsecondary scanning beams, characterized in that the lens position signal(LCE) is obtained by combination, in particular addition, of theprimary-beam error signal (CPP) and the secondary-beam error signal(OPP) 2) method according to claim 1, characterized in that the lensposition signal (LCE) is obtained by weighted combination of theprimary-beam error signal (CPP) and secondary-beam error signal (OPP).3) Method according to claim 2, characterized in that the lens positionsignal (LCE) is obtained from the primary-beam error signal (CPP) andthe secondary-beam error signal (OPP) in accordance with the followingrelationship: LCE=CPP−G*OPP, wherein the weighting factor (G) is chosenin such a way that the following holds true:${1 - {2G\quad {\cos \left( {\pi*\frac{\Delta \quad x}{p}} \right)}}} = 0$

wherein Ax designates the distance between the secondary beams and theprimary beam and p designates the track spacing of the optical recordingmedium (7). 4) Method according to claim 3, characterized in that theweighting factor (G) is set variably in a manner dependent on thedistance between the secondary beams and the primary beam and the trackspacing of the optical recording medium (7). 5) Method according to oneof claims 1-4, characterized in that, in order to form the lens positionsignal (LCE), a normalization is applied to the primary-beam errorsignal (CPP) and the secondary-beam error signal (OPP). 6) Methodaccording to one of the preceding claims, characterized in that the lensposition signal (LCE) is obtained from the primary-beam error signal(CPP) and the secondary-beam error signal (OPP) in accordance with thefollowing relationship:${{LCE} = {{{\left( {1 - G^{\prime}} \right)*{CPP}} - {G^{\prime}*{OPP}\quad {mit}\quad G^{\prime}}} = \frac{G}{\left( {1 + G} \right)}}},$

wherein G describes a weighting factor. 7) Method according to one ofthe preceding claims, characterized in that a track error signal (DPP)is additionally obtained by subtraction of the secondary-beam errorsignal (OPP) from the primary-beam error signal (CPP). 8) Methodaccording to claim 7, characterized in that the track error signal (DPP)is obtained from the primary-beam error signal (CPP) and thesecondary-beam error signal (OPP), in which the lens-movement-dependentcomponent tends to zero by setting a suitable compensation factor (K),formed according to the following relationship: DPP=CPP−K*OPP. 9) Methodaccording to claim 7 or 8, characterized in that, in order to form thetrack error signal (DPP), a normalization is applied to the primary-beamerror signal (CPP) and the secondary-beam error signal (OPP), and inthat this normalization is simultaneously used to form the lens positionsignal (LCE). 10) Method according to one of claims 7-9, characterizedin that the track error signal (DPP) is obtained from the primary-beamerror signal (CPP) and the secondary-beam error signal (OPP) inaccordance with the following relationship:${{DPP} = {{{\left( {1 - K^{\prime}} \right)*{CPP}} - {K^{\prime}*{OPP}\quad {mit}\quad K^{\prime}}} = \frac{K}{\left( {1 + K} \right)}}},$

wherein K describes a compensation factor. 11) Method for generating alens position signal, wherein the lens position signal (LCE) describesthe position of the optical axis (23) of an objective lens (6) of anapparatus for reading from and/or writing to an optical recording medium(7) with regard to the optical axis (22) of an optical scanner (21)assigned to the objective lens (6), wherein beams (15-18) incident ondifferent tracks of the recording medium (7) are generated and the beamsreflected from the recording medium (7) are detected, and wherein arespective error signal (OPP1, OPP2) is derived from the detectedreflected beams, characterized in that the lens position signal (LCE) isobtained by addition of the error signals (OPP1, OPP2). 12) Methodaccording to claim 11, characterized in that a first beam reflected fromthe optical recording medium (7) is detected by a first photodetector(11) having two detector areas and a second beam reflected from theoptical recording medium (7) is detected by a second photodetector (13)having two detector areas, in that a first error signal OPP1=E2−E1 and asecond error signal OPP2=F2−F1 are obtained from the output signals E1,E2 of the first photodetector (11) and the output signals F1, F2 of thesecond photodetector (13), respectively, and in that the lens positionsignal (LCE) is obtained by addition of the two error signals (OPP1,OPP2). 13) Method according to claim 11 or 12, characterized in that atleast two secondary beams are imaged onto the optical recording medium(7) with a distance of Δx=(2n−1)·p/2 where n=0, 1, 2 . . . to form anadditionally generated or imaginary primary beam, wherein p describesthe track spacing on the optical recording medium (7), and in that thelens position signal (LCE) is obtained by addition of the error signals(OPP1, OPP2) generated from the two secondary beams. 14) Methodaccording to claim 13, characterized in that a primary beam isadditionally generated on the recording medium (7) and the primary beamreflected from the optical recording medium (7) is detected in order toderive a corresponding error signal (CPP) from the detected reflectedprimary beam, and in that a direction signal (DIR) describing thedirection of movement of the objective lens (6) is derived from thephase between one of the error signals (OPP1, OPP2) derived for the twosecondary beams and the error signal (CPP) derived for the primary beam.15) Method according to claim 13 or 14, characterized in that a primarybeam is additionally generated on the recording medium (7) and theprimary beam reflected from the optical recording medium (7) isdetected, and in that a signal stating what type of track the primarybeam is presently scanning is derived from one of the error signals(OPP1, OPP2) derived from the two secondary beams. 16) Method accordingto one of claims 11-15, characterized in that, in order to form the lensposition signal (LCE), a normalization is applied to the respectiveerror signals (OPP1, OPP2). 17) Method according to one of claims 11-16,characterized in that two secondary scanning beam pairs and a primarybeam are generated, the secondary scanning beams and the primary beambeing incident on adjacent tracks of the optical recording medium (7),in that the secondary scanning beams reflected from the opticalrecording medium (7) and the reflected primary scanning beam aredetected and secondary-beam error signals (OPP1-OPP4) and a primary-beamerror signal (CPP) are derived therefrom, and in that the lens positionsignal (LCE) is derived by addition of the secondary-beam error signals(OPP1, OPP2) of one secondary beam pair, while a track error signal(DPP) is derived from the secondary-beam error signals (OPP3, OPP4) ofthe other secondary beam pair and the primary-beam error signal (CPP) byaddition of the corresponding secondary-beam error signals (OPP3, OPP4)and then weighted subtraction from the primary-beam error signal (CPP).18) Method according to claim 17, characterized in that a directionalsignal (DIR) describing the direction of movement of the objective lens(6) is derived from the phase between one of the secondary-beam errorsignals (OPP1-OPP4) and the primary-beam error signal (CPP). 19) Methodaccording to claim 17 or 18, characterized in that a signal yielding astatement about what type of track the primary beam is presentlyscanning is derived from one of the secondary-beam error signals(OPP1-OPP4), derived from the secondary beams. 20) Apparatus for readingfrom and/or writing to an optical recording medium, having a beamgenerating unit (1-3) for generating primary and secondary scanningbeams incident on adjacent tracks on the optical recording medium (7),having a photodetector unit (9) for detecting the primary and secondaryscanning beams reflected from the optical recording medium (7), andhaving an evaluation unit (10) for forming a primary-beam error signal(CPP) and a secondary-beam error signal (OPP) from the detected primaryand secondary scanning beams, characterized in that the evaluation unit(10) is configured in such a way that it generates a lens positionsignal (LCE), which describes the position of the optical axis of anobjective lens (6) of the apparatus with regard to the optical axis ofan optical scanner (21)—assigned to the objective lens (6)—of theapparatus, by combination, in particular addition, of the primary-beamerror signal (CPP) and secondary-beam error signal (OPP). 21) Apparatusaccording to claim 20, characterized in that the evaluation unit (10) isconfigured in such a way that it generates the lens position signal(LCE) in a manner dependent on the primary-beam error signal (CPP) andthe secondary-beam error signal (OPP) in accordance with the followingrelationship: LCE=CPP−G*OPP, wherein G describes a weighting factorwhich is chosen in such a way that the following holds true:${{1 - {2G\quad {\cos \left( {\pi*\frac{\Delta \quad x}{p}} \right)}}} = 0},$

wherein Δx describes the distance between the secondary beams and theprimary beam and p describes the track spacing on the optical recordingmedium (7). 22) Apparatus according to claim 20 or 21, characterized inthat the evaluation unit (10) is configured in such a way that, in orderto form the lens position signal (LCE), it applies a normalization tothe primary-beam error signal (CPP) and the secondary-beam error signal(OPP). 23) Apparatus according to one of claims 20-22, characterized inthat the lens position signal (LCE) is obtained from the primary-beamerror signal (CPP) and the secondary-beam error signal (OPP) inaccordance with the following relationship:${{LCE} = {{{\left( {1 - G^{\prime}} \right)*{CPP}} - {G^{\prime}*{OPP}\quad {where}\quad G^{\prime}}} = \frac{G}{\left( {1 + G} \right)}}},$

wherein G describes a weighting factor. 24) Apparatus according to oneof claims 20-23, characterized in that the evaluation unit (10) isconfigured in such a way that it additionally obtains a track errorsignal (DPP) by subtraction of the secondary-beam error signal (OPP)from the primary-beam error signal (CPP), in which track error signalthe lens-movement-dependent component tends to zero by setting asuitable compensation factor K, formed according to the followingrelationship: DPP=CPP−K*OPP. 25) Apparatus according to claim 24,characterized in that the evaluation unit (10) is configured in such away that, in order to form the track error signal (DPP), it applies anormalization to the primary-beam error signal (CPP) and thesecondary-beam error signal (OPP) and simultaneously uses thisnormalization to form the lens position signal (LCE). 26) Apparatusaccording to claim 24 or 25, characterized in that the track errorsignal (DPP) is obtained from the primary-beam error signal (CPP) andthe secondary-beam error signal (OPP) in accordance with the followingrelationship:${DPP} = {{{\left( {1 - K^{\prime}} \right)*{CPP}} - {K^{\prime}*{OPP}\quad {where}\quad K^{\prime}}} = \frac{K}{\left( {1 + K} \right)}}$

27) Apparatus for reading from and/or writing to an optical recordingmedium, having a beam generating unit (1-3) for generating beamsincident on different tracks on the optical recording medium (7), havinga photodetector unit (9) for detecting the beams reflected from theoptical recording medium (7), and having an evaluation unit (10) forgenerating error signals (OPP1, OPP2) corresponding to the reflectedbeams, characterized in that the evaluation unit (10) is configured insuch a way that it generates a lens position signal (LCE), whichdescribes the position of the optical axis of an objective lens (6) ofthe apparatus with regard to the optical axis of an optical scanner (21)assigned to the objective lens (6), by addition of the error signals(OPP1, OPP2). 28) Apparatus according to claim 27, characterized in thatthe beam generating unit (1-3) generates a first and a second beam, inthat the photodetector unit (9), for detecting the first and secondbeams reflected from the optical recording medium (7) respectively has aphotodetector (11, 13) having two detector areas, and in that theevaluation unit (10) is configured in such a way that it generates afirst error signal OPP1=E2−E1 from the output signals E1, E2 of thefirst photodetector (11) and a second error signal OPP2=F2−F1 from theoutput signals of the second photodetector (13) and obtains the lensposition signal (LCE) by addition of the two error signals OPP1+OPP2.29) Apparatus according to claim 27 or 28, characterized in that thebeam generating unit (1-3) generates a first secondary beam and a secondsecondary beam with a distance of Δx=(2n−1)·p/2, where n=0, 1, 2 . . . ,to an additionally generated or imaginary primary beam, p describing thetrack spacing on the optical recording medium (7), and in that theevaluation unit (10) is configured in such a way that it generates afirst error signal (OPP1) and a second error signal (OPP2) from thefirst and second secondary beams reflected from the optical recordingmedium (7) and obtains the lens position signal (LCE) by addition of thefirst and second error signals (OPP1, OPP2). 30) Apparatus according toclaim 29, characterized in that the beam generating unit (1-3) generatesa primary beam incident on the optical recording medium (7), and in thatthe evaluation unit (10) is configured in such a way that it generates aprimary-beam error signal (CPP) in a manner dependent on the primarybeam reflected from the optical recording medium (7), and derives adirection signal (DIR) which describes the direction of movement of theobjective lens (6), from the phase between the difference signal of theerror signals (OPP1, OPP2) generated for the two secondary beams and theprimary-beam error signal (CPP). 31) Apparatus according to claim 29 or30, characterized in that the beam generating unit (1-3) generates aprimary beam incident on the optical recording medium (7), and in thatthe evaluation unit (10) is configured in such a way that it derives,from one of the secondary-beam error signals (OPP1-OPP4) derived fromthe secondary beams, a signal containing a statement about what type oftrack the primary scanning beam is presently scanning. 32) Apparatusaccording to one of claims 27-31, characterized in that the evaluationunit (10) is configured in such a way that, in order to form the lensposition signal (LCE), it applies a normalization to the respectivesecondary-beam error signals (OPP1, OPP2). 33) Apparatus according toone of claims 27-32, characterized in that the beam generating unit(1-3) generates two secondary scanning beam pairs and a primary beam,the primary beam and the secondary beams being incident on adjacenttracks of the optical recording medium (7), and in that the evaluationunit (10) is configured in such a way that it derives a primary-beamerror signal (CPP) and corresponding secondary-beam error signals(OPP1-OPP4) from the primary and secondary scanning beams reflected fromthe optical recording medium (7), the evaluation unit (10) generatingthe lens position signal (LCE) by addition of the secondary-beam errorsignals (OPP1, OPP2) of one secondary beam pair, and generating a trackerror signal (DPP) by weighted subtraction of the sum of thesecondary-beam error signals (OPP3, OPP4) of the other secondary beampair from the primary-beam error signal (CPP). 34) Apparatus accordingto claim 33, characterized in that the evaluation unit (10) isconfigured in such a way that, in order to form the lens position signal(LCE), it applies a normalization to the primary-beam error signal (CPP)and the secondary-beam error signals (OPP1, OPP2) of one secondary beampair and, in order to form the track error signal (DPP), it applies anormalization to the primary-beam error signal (CPP) and thesecondary-beam error signals (OPP3, OPP4) of the other secondary beampair.